Begin with your hinge lego piece.  This is the piece that will enable you to attach an elastic cord to your flat lego plate.  Place a small drop of glue on the part that attaches to the plate.

Glue the hinge piece to the flat plate.

Now you can simply design and embellish your lego plate as you desire.

Use a piece of elastic cord to create your necklace.  Slip the cord through the hole in the lego piece.  Trim to length.

Here I am using lego tubes like beads - beading the necklace.

Tie the ends of your elastic cord together.

All done - your imagination and your LEGO collection are your only limitations.